1. Field of the Invention
This invention relates to a metallic acoustically treated sound absorbing panel having a reinforcing core of the honeycomb type so as to be adapted to withstand mechanical loading. The sound absorbing panel is manufactured to be advantageously capable of preventing galvanic corrosion and avoiding the effects of high temperature so as to prevent disbanding and structural failure. By way of example, the sound absorbing panel of this invention is strategically disposed at the inlet cowl of a jet engine to reduce the level of noise that is emitted from the engine into the environment.
2. Background Art
It is known to use sound absorbing panels within the inlet/exhaust system of a jet aircraft in order to reduce noise pollution which escapes from the engine into the environment. The conventional sound absorbing panels are typically manufactured from substrates of different metals (e.g. stainless steel and aluminum) that are adhesively bonded together during assembly. Because of the manner in which they are manufactured, the conventional sound absorbing panels have been susceptible to undesirable skin and structural failures which may lead to increased maintenance costs and to the possibility that the aircraft may have to be taken out of service.
More particularly, the use of dissimilar metal substrates that have a common interface or junction has been known to cause galvanic coupling which can result in corrosion. Moreover, the adhesive that is employed to bond the metal substrates together is exposed to the same high temperatures to which the exhaust system of the aircraft is exposed. Such corrosion and/or high temperatures correspondingly result in disbanding of the panel and the separation of the metal substrates. What is still more, during routine maintenance of the inlet system, a workman may have to walk over the sound absorbing panel. The weight of the workman and the lack of suitable structural reinforcement sometimes causes the conventional panel to break.
As a consequence of the foregoing, conventional sound absorbing panels are likely to fail which thereby necessitates an inefficient repair and/or replacement.
Disclosed below is a unique acoustically treated structurally reinforced sound absorbing panel that has particular application for use with the nacelle system (e.g. at the inlet cowl of a jet engine) of a jet aircraft to reduce the magnitude of the noise which escapes into the environment. The sound absorbing panel is manufactured so as to avoid structural failure, disbonding and galvanic corrosion, while being able to better withstand high temperature and mechanical loading.
The sound absorbing panel has a sound chamber comprising a reinforcing core of the honeycomb type that includes corrugated metallic (e.g. stainless steel) ribbons. An array of parallel aligned sound transmitting channels are established through the cells of the honeycomb core. A metallic (e.g. stainless steel) backskin at the bottom of the panel is attached to one side of the sound chamber. A perforated metallic (e.g. stainless steel) face sheet at the top of the panel is attached to the opposite side of the sound chamber. As an important improvement, a porous acoustic attenuation layer is (e.g. diffusion) bonded to the underside of the face sheet so as to lie below the perforations therein. The porous acoustic attenuation layer preferably comprises an arrangement of short metal (e.g. stainless steel) fibers that are held together by means of sintering.
In operation, incoming sound waves corresponding to the jet engine noise enter the sound chamber of the sound absorbing panel by way of the perforations formed in the face sheet and the porous acoustic attenuation layer bonded thereto. The incoming sound waves are then channeled through the parallel aligned cells of the sound chamber (e.g. the reinforcing honeycomb core) to be reflected between the face sheet and the backskin at opposite sides of the panel. In this regard, it should be appreciated that the metallic honeycomb core of the sound chamber serves the important dual functions of providing enhanced structural reinforcement to withstand mechanical loads and guiding the incoming and reflected sound waves between the face sheet and the backskin of the sound absorbing panel. The amplitude of the reflected sound waves that are repeatedly transmitted through the porous acoustic attenuation layer below the face sheet is successively attenuated, whereby to reduce the level of noise which escapes the sound chamber of the panel into the environment.